This invention relates to an automatic sample injector for injecting a liquid sample into the vaporization chamber of a gas chromatograph.
A syringe is commonly used for injecting a liquid sample to be vaporized for analysis into a gas chromatograph. Such a syringe comprises a barrel, a plunger adapted to slide inside the barrel in a liquid-tight relationship therewith, and a needle at the tip of the barrel having a liquid passage therethrough. After a liquid sample is introduced into the barrel, the needle is caused to penetrate a septum (a rubber membrane) and is inserted into the vaporization chamber such that the liquid sample can be dispersed. The liquid sample is made into particles as it is sprayed, vaporized by the heat in the vaporization chamber and transported into the chromatograph column by a carrier gas which is introduced into the vaporization chamber.
An automatic sample injector is for carrying out these processes automatically, comprising a syringe-driving mechanism for moving the syringe with respect to the vaporizer and inserting the needle into the vaporization chamber and a plunger-driving mechanism for moving the plunger with respect to the barrel so as to suck in a liquid sample into the barrel or to inject it out of the barrel. There may also be provided means for moving the syringe between a sample bottle and the vaporizer and for cleaning the barrel and the needle.
One of the problems with automatic sample injectors is that the plunger sometimes gets stuck inside the barrel and cannot be moved. A pulse motor is usually used for driving the plunger but since it is controlled in an open loop, it may appear to the motor (or the control unit therefor) as if the injector is operating normally even when the plunger is stuck inside the barrel and the injector is not operating normally at all. After a specified number of pulses is transmitted to the motor, for example, the control unit of the driving mechanism takes it for granted that the plunger has already moved and starts the next operation. If the plunger is stuck, the needle may be pulled out of the vaporization chamber before the liquid sample is completely injected thereinto, and subsequent processes will be carried out as if there was no abnormality. Since the automatic injector is used for analyzing many samples continuously, many analyses will thus be wasted in the case of such an accident. If the motor has a stronger torque than the force holding the plunger, there may be a damage to the plunger which is usually made of a metallic material such as stainless steel and is very thin (about .phi.1 mm.times.several cm).
In order to prevent such occurrences, it has been known to provide an automatic sample injector with a sensor for detecting the plunger at a specified home position (such as the position of the plunger when it is inserted most deeply) and to check whether the plunger has come back to the home position after a specified number of pulses has been transmitted from the control unit to the motor or whether the plunger has left the home position.
Although a prior art detector of this kind can detect an abnormal condition when the plunger does not return to the home position or leave the home position, this relates to a situation when the plunger is completely stuck to the barrel and cannot move at all. When the plunger is stuck but not completely and it can still move a little although the resistance is very strong, however, a prior art detector will not identify it as an abnormality, and there remains the possibility that the suction and injection of the sample may not be carried out as intended before the system moves onto the next process. If the motor has a strong torque, furthermore, the plunger is again likely to be damaged.